Although genetic mutations are now known to be a major cause of congenital cardiovascular malformation (CCVM), a significant proportion of CCVM is the result of nutritional deficiency or toxicity. Animal experimentation and epidemiologic studies have shown that folate deficiency is correlated with cardiac septal and aortic arch defects that fit the classification, i.e. they may be of neural crest origin. The mechanism of action of folate I prevention of congenital defects is not know. However, folate deficiency always is associated with an increase in the amino acid homocysteine (H[e]) and mothers of congenitally defective babies may have elevated H[e] even in the absence of low folate. They hypothesized that H[e] may be the teratogenic agent in folate deficiency, and that the mechanism may involve an effect upon the neural crest-derived ectomesenchyme. Exogenous H[e] was applied to avian embryos in ovo, inducing a specific set of cardiovascular and neural tube defects that simulate folate deficiency with high fidelity. Neural crest ectomesenchyme-derived vascular smooth muscle cells were treated with H[e] in vitro and responded with: an immediate Ca++ signal; protooncogene upregulation in 6 hr; and ultimately, cell proliferation. These effects appeared to be receptor-mediated, and could be blocked with NMDA receptor blockers. For this study they propose further tests of the above hypothesis, that will: 1. Characterize the phenotype of H[e]-induced congenital cardiovascular malformation in this model; 2.Determine the effect of H[e] on the migration, differentiation and matrix expression of the ectomesenchyme in vivo and in vitro; and 3. Begin to define the putative H[e] receptor and determine the effect of blocking this receptor. This study introduces a new model of a significant cause of CCVM and a novel concept of the role of H[e]. It has the potential to improve prevention of CCVM.